1. Field of the Invention
The invention relates to the field of chemical analyzers. More particularly, the invention relates to the field of chemical analyzers using electrochemical measuring systems. In still greater particularity, the invention relates to the field of chemical analyzers utilizing ion-sensing electrodes. By way of further characterization, but not by way of limitation thereto, the invention is a method for sensing ion concentration utilizing an ion-sensing electrode having a gas-permeable ion-impermeable membrane in which an electrolyte solution is pumped between the electrode and the membrane.
2. Description of the Related Art
U.S. Pat. Nos. 4,003,705 and 4,170,523 issued to E. E. Buzza et al. and assigned to the assignee of the present invention describe an electrochemical analysis apparatus for measuring both chloride and carbon dioxide in blood. As disclosed in those patents, the carbon dioxide-sensing element includes a renewable electrolyte disposed between a gas-permeable membrane and an ion-sensitive electrode. Gas diffusing through the membrane from a sample substance in a cup on the opposite side of the membrane from the electrode causes a reaction in the renewable electrolyte which is sensed by the ion electrode. The electrical output of this electrode, either as a rate or equilibrium condition, is proportional to the gas diffusing through the membrane.
In the above-mentioned patents, the sensor is located in communication with the cup containing a substrate with zero partial pressure or partial pressure of the gas to be detected equal to that in the renewable electrolyte, both, for instance, being in equilibrium to the atmosphere. When the sample is introduced into the substrate, the gas of interest is released, diffusing through the membrane, and initiating the reaction in the renewable electrolyte. Several factors may influence the rate of this initiation, such as the reproducibility and the time required for introducing the sample, the quality of stirring employed for mixing the sample with the substrate, and the time interval between renewing the electrolyte and introducing the sample.
In a situation, such as a stopped flow-through system, which requires a high throughput, initiation of the rate reaction by the previous method is not possible. The sample liquid must be premixed with acid, thereby releasing the ion of interest before entering the flow cell where the sensor is located. Thus, the addition of sample is not the event which initiates the rate reaction. Also, the premixed sample is purged by a bicarbonate solution of fixed concentration which is also premixed with acid to act as an internal reference. At no time, therefore, is there a solution of zero or partial pressure of CO.sub.2 equal to that of the renewable electrolyte in contact with the outside of the membrane. In this system there is no stirring required in the flow cell during the measuring interval because the sample and acid have been premixed.